This invention relates generally to electrical power conversion methods and circuits, and more particularly to inductors with reduced AC losses.
Current source inverters are typical building blocks for many power converters. This kind of inverter typically includes a voltage inverter (e.g., having a bridge/half bridge configuration) and a current defining element, such as an AC choke. Typical applications include series resonant converters where the current source inverter represents a substantial part of the primary circuitry. An inductive choke is also a part of the inverter and typically consists of a magnetic core and a winding.
The energy density of a magnetic field uB is defined by equation (1). In order to achieve a compact and a high density inductor, a volume with a maximum uB may ideally be created.uB=(1/2μ)B2  (1)
A magnetic core provides significant assistance in achieving the high energy density because the flux (represented by its flux density B) can be shaped and concentrated, and can flow through a low permeability space. This may be rather difficult to achieve in, e.g., an air inductor. For high frequency applications, a ferrite is conventionally the material used, examples of which may include soft ferrites such as manganese-zinc ferrite, nickel-zinc ferrites, lithium-zinc ferrites, and the like. Based on the selected material, frequency and target core loss density, the amplitude of the flux density B (ΔB respectively) can be derived. This yields a certain maximum flux density B limiting the magnetic field uB. Looking again to equation (1), a second possibility for increasing the energy density is the permeability μ of the space where the flux B·A (A=cross section area) is allowed to flow. Generally speaking, for smaller permeability, the obtained energy density may correspondingly be larger. The best results are therefore achieved with air because its permeability approaches μ0. Taking into account some real numbers (Ba=50 mT, μ0=4π*10−7 H/m), equation (1) yields 995 J/m3 which approximately represents an energy density limit for a typical magnetic device with a ferrite core operated at a flux density of 50 mT. One of skill in the art may further appreciate that the stored energy is in the field of the air gap (μ=1) and not in the core (μ>>1).
Because the current through an inductive choke typically contains a significant AC component, a litz wire is often used to wind a coil so that skin and proximity effects are minimized. However, litz wires are generally not suitable for planar structures where the winding is, e.g., integrated into a multilayer printed circuit board (“PCB”) as copper planes or copper tracks. These objects suffer from AC losses due to eddy currents which are mainly caused by: 1) a fringing field from the air gap; and 2) the skin and proximity effects due to other windings. Because of this, the space where the energy is stored must have sharp boundaries, or otherwise the field 24 leaks out and can cause eddy currents in windings as represented for example in FIG. 10.
With exemplary reference to FIGS. 1 and 2, those of skill in the art have previously implemented a distributed gap 13 with respect to a core 11 and one or more conductors 12 in planar magnetics as an efficient way to achieve a controlled behavior for the fringing field. The problem of the uniformly distributed gap is that for high frequency applications a ferrite base material would need to be used in conjunction with a resin. However, ferrite base materials have relatively high associated losses, and other standard distributed gap materials do not provide sufficiently low losses when used at high frequencies.
Referring now to FIGS. 3 and 4, an alternative group of solutions as previously known in the art propose a structure 10 with a uniformly distributed gap 13, or a “quasi-distributed air gap”. In other words, an otherwise distributed air gap may be divided into multiple smaller gaps.
Referring next to FIG. 5, another conventional solution for minimizing AC losses in planar inductors and integrated magnetics 10 includes a core 11 which is suitable for high current low frequency applications where the gap is filled with a composite material 13 made of high saturation low frequency powder material. More specifically, such a structure may be suitable for converters with integrated magnetics and matrix integrated magnetics (MIMs).
Referring next to FIG. 6, a storage magnetic element is known in the art which seeks to minimize the power loss in the planar winding due to the fringing magnetic field associated with a discrete air gap. A magnetic core 11 is formed by an E section 11b made of high permeability magnetic material and an I section 11a made by a material capable of storing energy due to its distributed gap structure 13. This kind of design also requires a distributed gap material which is usually suitable only for high flux and low frequency applications.
Referring next to FIG. 7, another conventional application of a magnetic core includes a substrate with magnetically permeable material that has a first region 15a and a second region 15b near the first region. Support is provided to maintain a juxtaposition between the first region and the second region, and a slit 16 is formed through the magnetically permeable material between the first region and the second region. A binding agent is introduced into the slit and the support may then be removed, wherein the binding agent maintains the juxtaposition between the first region and the second region after the support is removed. This process is single sided, and therefore energy storage is limited unless physical dimensions are enlarged. Therefore, the final component may be suitable for rather small power applications.
Referring next to FIG. 8, another example of a planar magnetic element may include a substrate 17 and a protection layer 18, with magnetic layers 19, insulators 20 and planar coils 12 disposed between the substrate and the protection layer. However, magnetic layers are only with difficulty included in the PCB, and therefore this approach is not eligible for cost optimized high density PCB based converters.
Referring still further to FIG. 9, a single piece core structure for magnetic components is known in the art which does not require insulating spacer materials and bonding materials. This approach includes a monolithic core structure 10 fabricated from a magnetic material 22, a gap 21 integrally formed in the body and a conductive element 12 establishing a conductive path configured for surface mount termination. This design for surface mount components may typically be used in low voltage high current point of load (“POL”) converters. There are numerous potential problems with such a configuration, however. For example, the monolithic structure of the core does not allow the use of planar PCB windings, an important factor for compact converters because the PCB windings cannot be inserted into the monolithic core structure. Any other ways to build a high voltage winding (e.g., for 400V applications) may generally be difficult and/or unreliable due to the risk of isolation breakdown. Further, the manufacturing process for such a structure requires a certain minimum thickness of the non-magnetic ceramic layer for the gap (21), which is less than optimal to the formation of energy storage geometry as discussed herein, supra. The construction for the conductive element is also unsuitable for high frequency operation because the skin effects may generally lead to reduced copper usage and therefore significant power losses. Finally, the structure does not typically allow for inductance trimming as essential for certain type of converters, where the core is sintered together with the gap and therefore a smaller tolerance of inductance cannot be achieved.
The aforementioned solutions as conventionally known in the art for dealing with the fringing magnetic field may each therefore be characterized as suffering from one or more following problems: (a) the energy storage density is low, (b) the inductance cannot be trimmed, or (c) a uniformly distributed gap material for low frequency high flux density is used.
It would therefore be desirable to provide a compact high density planar converter which adequately targets each of these features.